Since the development of high-pressure boilers, operating at both sub-critical and super-critical pressures, the origin, formation and effects of internal deposits have been a vital concern to the designer. Although, for sustained operation, an extremely limited degree of internal scale formation is desired to protect the base metal from oxidation, the deposition of excess solid products in porous form from the flowing fluid and in a unique wave-like pattern can result in increased frictional and heat transfer resistances and the requirement for frequent cleaning of internal surfaces. This requirement of internal cleaning results in lower availability and costly outage time but is necessary if tube overheating, particularly in the high heat absorbing zones of the furnace, and forced outages are to be prevented. Efforts have been concentrated in the areas of cycle design and water treatment in an attempt to minimize the deposition of water-bearing soluble constituents. Presently, condensate polishing demineralizers are used for total solids control, and dissolved oxygen and carbon dioxide are controlled by deaeration and hydrazine treatment, but the gradual buildup of internal deposits persists. It has been found that even with optimum boiler water conditions, the basic iron and water reaction takes place continuously within a closed cycle. This reaction is a continuous process of deposition, release, transport and redeposition of corrosion products generated within the steam generating unit, in addition to the dissolved solids carried into the steam generating unit from external sources.